High density memory module using stacked printed circuit boards

ABSTRACT

A module is electrically connectable to a computer system. The module includes a frame having an edge connector with a plurality of electrical contacts which are electrically connectable to the computer system. The module further includes a first printed circuit board coupled to the frame. The first printed circuit board has a first surface and a first plurality of components mounted on the first surface. The first plurality of components is electrically coupled to the electrical contacts of the edge connector. The module further includes a second printed circuit board coupled to the frame. The second printed circuit board has a second surface and a second plurality of components mounted on the second surface. The second plurality of components is electrically coupled to the electrical contacts of the edge connector. The second surface of the second printed circuit board faces the first surface of the first printed circuit board. The module further includes at least one thermally conductive layer positioned between the first plurality of components and the second plurality of components. The at least one thermally conductive layer is thermally coupled to the first plurality of components, to the second plurality of components, and to the electrical contacts of the edge connector.

CLAIM OF PRIORITY

The present application claims the benefit of U.S. ProvisionalApplication No. 60/561,009, filed Apr. 9, 2004, and U.S. ProvisionalApplication No. 60/589,777, filed Jul. 21, 2004, each of which isincorporated in its entirety by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to computer modules having aplurality of components mounted on two or more stacked printed circuitboards, and more specifically to high density memory modules usingstacked printed circuit boards with heat dissipation structures.

2. Description of the Related Art

Computer systems often utilize modules comprising one or more printedcircuit boards (PCBs). Each PCB has one or more components (e.g.,integrated circuits or ICs) mounted thereon, and the components can bemounted on one side or on both sides of the PCB. In certain computersystems, the PCBs of the module are stacked next to one another toincrease the functionality of the module. For example, board stacking isa method used to increase the memory density in memory subsystems. Thetechnique is also used to increase the device density of othercomponents, such as logic. Stacking enhances the capability of themodule, particularly if components are assembled on each of the twosides of each of the stacked PCBs. In such configurations, thecomponents mounted on one side of one PCB are positioned in closeproximity to the components mounted on a neighboring side of aneighboring PCB.

Stacking configurations can cause problems due to power dissipation inthe components which are in close proximity. Some or all of thecomponents can generate significant amounts of heat, which can raise thetemperature of the component itself or of the surrounding components ofthe module. The narrow air gap between the components on either side ofthe stacked PCBs prevents air flow which would otherwise keep thecomponents within their specified operating temperature ranges. Theraised temperature of these components can have harmful effects on theperformance of the components, causing them to malfunction.

Prior art systems utilize heat spreaders to radiate the heat away fromthe heat-generating component and away from the surrounding componentsof the module. Such prior art heat spreaders are mounted over theheat-generating components. In stacked configurations, the prior artheat spreaders are typically mounted over components on an outsidesurface of the PCB (i.e., a surface away from a neighboring PCB). Whilethese prior art heat spreaders can dissipate heat generated by thecomponents on the outside surface of the PCB, components on the insidesurfaces would remain hot. In addition, the components on the outsidesurface of the PCB are effectively cooled by air flowing across thecomponents from a ventilation fan. However, the narrow air gap betweenthe stacked PCBs would allow very little cool air from the ventilationfan to cool the components on the inside surfaces to within thespecified operating temperatures.

SUMMARY OF THE INVENTION

In certain embodiments, a module is electrically connectable to acomputer system. The module comprises a frame having an edge connectorwith a plurality of electrical contacts which are electricallyconnectable to the computer system. The module further comprises a firstprinted circuit board coupled to the frame. The first printed circuitboard has a first surface and a first plurality of components mounted onthe first surface. The first plurality of components is electricallycoupled to the electrical contacts of the edge connector. The modulefurther comprises a second printed circuit board coupled to the frame.The second printed circuit board has a second surface and a secondplurality of components mounted on the second surface. The secondplurality of components is electrically coupled to the electricalcontacts of the edge connector. The second surface of the second printedcircuit board faces the first surface of the first printed circuitboard. The module further comprises at least one thermally conductivelayer positioned between the first plurality of components and thesecond plurality of components. The at least one thermally conductivelayer is thermally coupled to the first plurality of components, to thesecond plurality of components, and to the electrical contacts of theedge connector.

In certain embodiments, a module is connectable to a computer system.The module comprises a frame comprising an edge connector connectable tothe computer system. The module further comprises a first printedcircuit board coupled to the frame. The first printed circuit board hasa first surface and a first plurality of components mounted on the firstsurface. The first plurality of components is electrically coupled tothe edge connector. The module further comprises a second printedcircuit board coupled to the frame. The second printed circuit board hasa second surface and a second plurality of components mounted on thesecond surface. The second plurality of components is electricallycoupled to the edge connector. The second surface faces the firstsurface. The module further comprises a heat spreader comprising atleast one sheet of thermally conductive material. The heat spreader ispositioned between and thermally coupled to the first plurality ofcomponents and the second plurality of components. The heat spreader isthermally coupled to the edge connector.

In certain embodiments, a method conducts heat away from a firstplurality of components mounted on a first surface of a first printedcircuit board and from a second plurality of components mounted on asecond surface of a second printed circuit board. The method comprisescoupling the first printed circuit board and the second printed circuitboard to a frame comprising an edge connector. The first surface facesthe second surface. The method further comprises positioning a thermallyconductive layer between the first plurality of components and thesecond plurality of components. The method further comprises thermallycoupling the thermally conductive layer to the first plurality ofcomponents, to the second plurality of components, and to the edgeconnector. The method further comprises electrically and thermallycoupling the edge connector to a computer system. A thermal pathway isprovided for heat to be removed from the first plurality of componentsand from the second plurality of components to the computer systemthrough the edge connector.

In certain embodiments, a method fabricates a module electricallyconnectable to a computer system. The method comprises providing a framecomprising an edge connector electrically connectable to the computersystem and at least one layer of thermally conductive material which isthermally coupled to the edge connector. The method further comprisesmounting a first printed circuit board to the frame. The first printedcircuit board has a first surface and a first plurality of componentsmounted on the first surface. The first plurality of components iselectrically coupled to the edge connector and is thermally coupled tothe at least one layer of thermally conductive material. The methodfurther comprises mounting a second printed circuit board to the frame.The second printed circuit board has a second surface and a secondplurality of components mounted on the second surface. The secondplurality of components is electrically coupled to the edge connectorand is thermally coupled to the at least one layer of thermallyconductive material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a cross-sectional view of a module inaccordance with certain embodiments described herein.

FIG. 2 schematically illustrates a cross-sectional view of an exemplaryframe having multiple printed circuit boards (PCBs) in accordance withcertain embodiments described herein.

FIGS. 3A-3C schematically illustrate an exemplary process for forming ariser PCB in accordance with certain embodiments described herein.

FIG. 4A schematically illustrates one side of an exemplary first PCBcompatible with the exemplary frame schematically illustrated by FIG. 2.

FIG. 4B schematically illustrates one side of an exemplary second PCBcompatible with the exemplary frame schematically illustrated by FIG. 2.

FIG. 5 schematically illustrates an exemplary module with the exemplaryframe of FIG. 2, a first PCB with a first plurality of components on afirst surface, and a second PCB with a second plurality of components ona second surface facing the first surface.

FIG. 6 schematically illustrates an exemplary module with the exemplaryframe of FIG. 2, a first PCB with a first plurality of components on twosurfaces, and a second PCB with a second plurality of components on twosurfaces.

FIG. 7 is a flowchart of an exemplary method of fabricating a modulewhich is electrically connectable to a computer system in accordancewith certain embodiments described herein.

FIG. 8 is a flowchart of providing the frame in accordance with certainembodiments described herein.

FIG. 9 is a flowchart of mounting the first PCB to the frame inaccordance with certain embodiments described herein.

FIG. 10 is a flowchart of mounting the second PCB to the frame inaccordance with certain embodiments described herein.

FIGS. 11A-11C schematically illustrate exemplary PCBs with holes whichfit onto corresponding pins of a jig.

FIG. 12 is a flowchart of an exemplary fabrication method using a jighaving pins corresponding to the holes of the PCBs schematicallyillustrated by FIGS. 11A-11C.

FIG. 13 schematically illustrates a side view of exemplary electricalconnections between the electrical contacts of the base PCB, theelectrical contacts of the first riser PCB, and the electrical contactsof the first PCB.

FIG. 14 schematically illustrates an exemplary module having a thermallyconductive piece which is positioned on the module along a portion ofthe opposite edge away from the edge connector.

FIG. 15 schematically illustrates an exemplary frame having a firstportion and a second portion in accordance with embodiments describedherein.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 schematically illustrates a cross-sectional view of a module 10in accordance with certain embodiments described herein. The module 10comprises a frame 20 having an edge connector 22 with a plurality ofelectrical contacts 24 which are electrically connectable to a computersystem (not shown). The module 10 further comprises a first printedcircuit board (PCB) 30 coupled to the frame 20. The first PCB 30 has afirst surface 32 and a first plurality of components 34 mounted on thefirst surface 32 and electrically coupled to the electrical contacts 24of the edge connector 22. The module 10 further comprises a second PCB40 coupled to the frame 20. The second PCB 40 has a second surface 42and a second plurality of components 44 mounted on the second surface 42and electrically coupled to the electrical contacts 24 of the edgeconnector 22. The second surface 42 of the second PCB 40 faces the firstsurface 32 of the first PCB 30. The module 10 further comprises at leastone thermally conductive layer 50 positioned between the first pluralityof components 34 and the second plurality of components 44. The at leastone thermally conductive layer 50 is thermally coupled to the firstplurality of components 34, to the second plurality of components 44,and to the electrical contacts 24 of the edge connector 22.

The frame 20 of certain embodiments comprises the edge connector 22 withthe plurality of electrical contacts 24, and further comprises aplurality of electrical contacts 26 (e.g., pads or solder balls) whichare electrically connectable to the first PCB 30 and the second PCB 40.In addition, the frame 20 of certain embodiments provides electricalconduits 28 from the electrical contacts 26 to the electrical contacts24 of the edge connector 22. In certain embodiments, the electricalcontacts 24 of the edge connector 22 are configured to be electricallyconnected to a corresponding socket of a PCB (e.g., motherboard) of thecomputer system. In certain embodiments, the electrical contacts 24 areon a single side of the frame 20, while in other embodiments, theelectrical contacts 24 are on both sides of the frame 20, asschematically illustrated by FIG. 1. Exemplary materials for theelectrical contacts 24, 26 and the electrical conduits 28 compatiblewith embodiments described herein include, but are not limited to,aluminum, copper, gold-plated copper, and other conductive metals andalloys. Persons skilled in the art can select appropriate materials andconfigurations for the electrical contacts 24 of the edge connector 22and the corresponding socket in accordance with embodiments describedherein. In addition, persons skilled in the art can select appropriatematerials and configurations of the electrical contacts 26 andelectrical conduits 28 in accordance with embodiments described herein.

In certain embodiments, the frame 20 further comprises the at least onethermally conductive layer 50 which is thermally coupled to the edgeconnector 22. In certain embodiments, the at least one thermallyconductive layer 50 comprises copper (e.g., “two-ounce” copper sheetcorresponding to an areal density of two ounces per square foot),aluminum, carbon, or another sufficiently thermally conductive material.In certain embodiments, the at least one thermally conductive layer 50is substantially electrically conductive, while in other embodiments,the at least one thermally conductive layer 50 is substantiallyelectrically insulative. While the embodiment schematically illustratedby FIG. 1 has one thermally conductive layer 50, other embodiments havetwo, three, four, or more thermally conductive layers 50. Generally, thethermal conductivity of the at least one thermally conductive layer 50increases with increasing thickness of the at least one thermallyconductive layer 50. The thickness of an exemplary thermally conductivelayer 50 comprising copper is approximately 0.2 millimeter. Personsskilled in the art can select appropriate materials, thicknesses, andconfigurations for the at least one thermally conductive layer 50 inaccordance with embodiments described herein.

In certain embodiments, the frame 20 comprises one or more PCBs whichprovide electrical conductivity from the edge connector 22 to the firstPCB 30 and to the second PCB 40. One or more of the PCBs of the frame 20of certain embodiments are multilayer structures formed by epoxylamination of layers of electrically insulative materials andelectrically conductive materials which form conductive traces, groundplanes, voltage planes, embedded passive components, and vias. Examplesof electrically insulative materials compatible with embodimentsdescribed herein include, but are not limited to, plastic, polyimide,fiberglass (e.g., FR4 material), and other dielectric materials.Examples of electrically conductive materials compatible withembodiments described herein include, but are not limited to, conductivepolymers, conductive inks, copper, aluminum, and other metals andalloys. In certain embodiments, the electrically conductive material isdeposited onto a dielectric layer (e.g., by copper clad processes as areknown to persons skilled in the art). Persons skilled in the art canselect appropriate materials and techniques to fabricate PCBs compatiblewith embodiments described herein.

FIG. 2 schematically illustrates a cross-sectional view of an exemplaryframe 20 having multiple PCBs in accordance with certain embodimentsdescribed herein. In certain embodiments, the frame 20 comprises a basePCB 60, a first riser PCB 70, and a second riser PCB 80. Certainembodiments of the frame 20 comprise fewer than three PCBs, while otherembodiments comprise more than three PCBs.

As schematically illustrated by FIG. 2, in certain embodiments, the basePCB 60 comprises the edge connector 22 and two thermally conductivelayers 50 a, 50 b on either side of a dielectric layer 61. In certainembodiments in which the two thermally conductive layers 50 a, 50 b areelectrically conductive, the dielectric layer 61 electrically insulatesthe two thermally conductive layers 50 a, 50 b from one another. Thebase PCB 60 of certain embodiments provides thermal conductivity betweenthe thermally conductive layers 50 a, 50 b and the edge connector 22. Incertain such embodiments, one thermally conductive layer 50 a isthermally coupled to a first set of electrical contacts 24 of the edgeconnector 22 and the other thermally conductive layer 50 b is thermallycoupled to a second set of electrical contacts 24 of the edge connector22.

The base PCB 60 of certain embodiments further comprises a firstplurality of electrical contacts 62 at a first surface 63 of the basePCB 60 which are electrically coupled to the edge connector 22 byelectrical conduits 68. The base PCB 60 of certain embodiments alsocomprises a second plurality of electrical contacts 64 at a secondsurface 65 of the base PCB 60 which are electrically coupled to the edgeconnector 22 by electrical conduits 69.

The first riser PCB 70 of certain embodiments comprises a thirdplurality of electrical contacts 72 which are electrically coupled tothe first plurality of electrical contacts 62 of the base PCB 60 andwhich are electrically connectable to the first PCB 30. As describedmore fully below, the first riser PCB 70 has a thickness selected tospace the first surface 32 of the first PCB 30 at a sufficient distanceaway from the base PCB 60 so that the first plurality of components 34fit between the first surface 32 of the first PCB 30 and the at leastone thermally conductive layer 50 of the base PCB 60. Similarly, thesecond riser PCB 80 of certain embodiments comprises a fourth pluralityof electrical contacts 82 which are electrically coupled to the secondplurality of electrical contacts 64 of the base PCB 60 and which areelectrically connectable to the second PCB 40. As described more fullybelow, the second riser PCB 80 has a thickness selected to space thesecond surface 42 of the second PCB 40 at a sufficient distance awayfrom the base PCB 60 so that the second plurality of components 44 fitbetween the second surface 42 of the second PCB 40 and the at least onethermally conductive layer 50 of the base PCB 60.

In certain embodiments, the first riser PCB 70 is formed by a processschematically illustrated by FIGS. 3A-3C. A PCB 90 is provided in whichholes 92 are formed through the thickness of the PCB 90. The holes 92are formed generally along an edge 94 of the PCB 90, as schematicallyillustrated by FIG. 3A. Persons skilled in the art can selectappropriate methods of forming the holes 92 (e.g., laser drilling) inaccordance with embodiments described herein. A plating layer 95 of aconductive material (e.g., copper) is then applied to the inside surfaceof each hole 92 and to an area on a top surface 96 of the PCB 90, asschematically illustrated by FIG. 3B, and to an area on a bottom surface97 of the PCB 90. The portions of the plating layer 95 corresponding tothe holes 92 are electrically insulated from one another (e.g., byspaces 98). Persons skilled in the art can select appropriate materialsand methods (e.g., copper cladding, laser removal of extraneous platingmaterial) for forming the plating layer 95 in accordance withembodiments described herein. The PCB 90 is then cut along a linegenerally parallel to the edge 94 and across the holes 92 (e.g., thedashed line of FIG. 3B). As schematically illustrated by FIG. 3C, theplated and cut holes 92 of the resultant structure of the first riserPCB 70 form the third plurality of electrical contacts 72 which areelectrically coupled to the first plurality of electrical contacts 62 ofthe base PCB 60 and which are electrically connectable to the first PCB30. A similar process is used to form the second riser PCB 80 and thefourth plurality of electrical contacts 82 in certain embodiments.

FIG. 4A schematically illustrates one side of an exemplary first PCB 30compatible with the exemplary frame 20 schematically illustrated by FIG.2. In certain embodiments, the first PCB 30 comprises a plurality ofelectrical contacts 36 along an edge 37 of the first PCB 30. Theplurality of electrical contacts 36 are electrically coupled to aplurality of component contacts 38 which are connectable to the firstplurality of components 34. In certain embodiments, the plurality ofelectrical contacts 36 of the first PCB 30 are electrically connectableto the third plurality of electrical contacts 72 of the first riser PCB70. In certain embodiments, the first PCB 30 is configured to havecomponents 34 only on one side, while in other embodiments, the firstPCB 30 is configured to have components 34 on both sides.

Similarly, as schematically illustrated by FIG. 4B, an exemplary secondPCB 40 comprises a plurality of electrical contacts 46 along an edge 47of the second PCB 40, with the plurality of electrical contacts 46electrically coupled to a plurality of component contacts 48 which areconnectable to the second plurality of components 44. In certainembodiments, the plurality of electrical contacts 46 of the second PCB40 are electrically connectable to the fourth plurality of electricalcontacts 82 of the second riser PCB 80. In certain embodiments, thesecond PCB 40 is configured to have components 44 only on one side,while in other embodiments, the second PCB 40 is configured to havecomponents 44 on both sides.

In certain embodiments, the first plurality of components 34 and/or thesecond plurality of components 44 comprises integrated circuits havingpackaging which include but are not limited to, thin small-outlinepackage (TSOP), ball-grid-array (BGA), fine-pitch BGA (FBGA), micro-BGA(μBGA), mini-BGA (mBGA), and chip-scale packaging (CSP). Memorycomponents 34, 44 compatible with embodiments described herein,including but not limited to, random-access memory (RAM), dynamicrandom-access memory (DRAM), synchronous DRAM (SDRAM), anddouble-data-rate DRAM (e.g., DDR-1, DDR-2, DDR-3). In certain suchembodiments, as schematically illustrated by FIGS. 4A and 4B, thecomponent contacts 38, 48 are configured to be electrically connected tomemory devices having BGA packaging. In addition, the components 34, 44of certain embodiments further comprise other types of integratedcircuits or electrical components, including, but not limited to,registers, clocks, and microprocessors. In certain embodiments, at leastsome of the components 34 of the first PCB 30 are stacked (e.g., packagestacked or die stacked) on one another, while in other embodiments, thecomponents 34 of the first PCB 30 are not stacked on one another. Incertain embodiments, at least some of the components 44 of the secondPCB 40 are stacked (e.g., package stacked or die stacked), while inother embodiments, the components 44 of the second PCB 40 are notstacked.

FIG. 5 schematically illustrates an exemplary module 10 with theexemplary frame 20 of FIG. 2, a first PCB 30 with a first plurality ofcomponents 34 on a first surface 32, and a second PCB 40 stacked withthe first PCB 30. The second PCB 40 has a second plurality of components44 on a second surface 42 facing the first surface 32. In certainembodiments, the first PCB 30 and the second PCB 40 are generallyparallel to one another, while in other embodiments, the first PCB 30and the second PCB 40 have a non-zero angle therebetween.

The first PCB 30 of FIG. 5 has component contacts 38 at the firstsurface 32 which are electrically connected to the correspondingcomponents 34 and to the electrical contacts 36 at the end of the firstPCB 30. As schematically illustrated by FIG. 5, the electrical contacts36 of the first PCB 30 are electrically coupled to the electricalcontacts 24 of the edge connector 22 through the electrical contacts 72of the first riser PCB 70, through the electrical contacts 62 of thebase PCB 60, and through the electrical conduits 68 of the base PCB 60.Similarly, the second PCB 40 of FIG. 5 has component contacts 48 at thesecond surface 42 which are electrically connected to the correspondingcomponents 44 and to the electrical contacts 46 at the end of the secondPCB 40. As schematically illustrated by FIG. 5, the electrical contacts46 of the second PCB 40 are electrically coupled to the electricalcontacts 24 of the edge connector 22 through the electrical contacts 82of the second riser PCB 80, through the electrical contacts 64 of thebase PCB 60, and through the electrical conduits 69 of the base PCB 60.

In the embodiment schematically illustrated by FIG. 5, the firstplurality of components 34 are thermally coupled to the thermallyconductive layer 50 a which is thermally coupled to a portion of theelectrical contacts 24 of the edge connector 22. Similarly, the secondplurality of components 44 are thermally coupled to the thermallyconductive layer 50 b which is thermally coupled to a portion of theelectrical contacts 24 of the edge connector 22.

In certain embodiments, at least some of the components 34, 44 are incontact with the least one thermally conductive layer 50, while in otherembodiments, at least some of the components 34, 44 are spaced away fromthe at least one thermally conductive layer 50. In certain embodiments,the thickness of the first riser PCB 70 is selected to position thefirst surface 32 of the first PCB 30 at a desired distance from thethermally conductive layer 50 a. Similarly, in certain embodiments, thethickness of the second riser PCB 80 is selected to position the secondsurface 42 of the second PCB 40 at a desired distance from the thermallyconductive layer 50 b. These distances between the at least onethermally conductive layer 50 and the first surface 32 and the secondsurface 42 are selected to provide sufficient thermal conductivitybetween the components 34, 44 and the at least one thermally conductivelayer 50.

In certain embodiments, the at least one thermally conductive layer 50comprises a layer of a thermally conductive grease 50 c which contactsat least some of the components 34, 44 and a corresponding one of the atleast one thermally conductive layers 50 a, 50 b. In certain suchembodiments, the thermally conductive grease provides an improvedthermal connection with the components 34, 44, thereby improving theheat transfer away from the components 34, 44. Persons skilled in theart can select an appropriate thermally conductive grease 50 c inaccordance with embodiments described herein.

Upon connection of the exemplary module 10 schematically illustrated byFIG. 5 to a socket of a computer system motherboard, the module 10provides a path for heat transfer from the first plurality of components34, through the thermally conductive grease 50 c and the thermallyconductive layer 50 a, through the contacts 24 of the edge connector 22,to the motherboard. Similarly, the module 10 provides a path for heattransfer from the second plurality of components 44, through thethermally conductive grease 50 c and the thermally conductive layer 50b, through the contacts 24 of the edge connector 22, to the motherboard.By providing a thermal path from the components 34, 44 through the edgeconnector 24 to the motherboard, certain embodiments advantageously donot utilize a separate thermal connection to other portions of thecomputer system (e.g., the chassis or enclosure) which may beinaccessible for this purpose. In addition, certain embodimentsadvantageously do not utilize separate heat spreaders on the outsidesurface of the module 10 which would otherwise increase the width of themodule 10. The at least one thermally conductive layer 50, along withthe electrical connections 24 of the edge connector 24 thereby serve asa heat spreader to dissipate heat from the components 34, 44.

FIG. 6 schematically illustrates an exemplary module 10 with theexemplary frame 20 of FIG. 2, a first PCB 30 with a first plurality ofcomponents 34 on two surfaces, and a second PCB 40 with a secondplurality of components 44 on two surfaces. The first PCB 30 of FIG. 6has component contacts 38 at both surfaces which are electricallyconnected to the corresponding components 34 and to the electricalcontacts 36 at the end of the first PCB 30. As schematically illustratedby FIG. 6, in certain embodiments, the components 34 on the firstsurface 32 of the first PCB 30 are thermally coupled to the thermallyconductive layer 50 a through a layer of thermally conductive grease 50c. In certain embodiments, the components 34 on the opposite surface ofthe first PCB 30 are not thermally coupled to the at least one thermallyconductive layer 50, while in other embodiments, the components 34 onthe opposite surface of the first PCB 30 are thermally coupled to the atleast one thermally conductive layer 50. Similarly, the second PCB 40 ofFIG. 6 has component contacts 48 at both surfaces which are electricallyconnected to the corresponding components 44 and to the electricalcontacts 46 at the end of the first PCB 40. As schematically illustratedby FIG. 6, in certain embodiments, the components 44 on the secondsurface 42 of the second PCB 40 are thermally coupled to the thermallyconductive layer 50 b through a layer of thermally conductive grease 50c. In certain embodiments, the components 44 on the opposite surface ofthe second PCB 40 are not thermally coupled to the at least onethermally conductive layer 50, while in other embodiments, thecomponents 44 on the opposite surface of the second PCB 40 are thermallycoupled to the at least one thermally conductive layer 50.

Certain embodiments described herein advantageously provide stacked PCBswith improved thermal dissipation properties. Certain embodimentsdescribed herein advantageously provide memory modules with increasedmemory capacity while keeping the thickness of the memory module below apredetermined value. For example, for certain embodiments withcomponents 34, 44 comprising DDR2 DRAM integrated circuits with BGApackaging on both sides of each of the first PCB 30 and the second PCB40, the module 10 has a thickness of less than approximately 5.6millimeters. Thus, certain embodiments advantageously allow use of themodule 10 in cramped spaces. Certain embodiments advantageously reducethe cost of ventilation of the module 10. Certain embodimentsadvantageously maintain temperatures of the components 34, 44 within adesired operational temperature range.

FIG. 7 is a flowchart of an exemplary method 100 of fabricating a module10 which is electrically connectable to a computer system in accordancewith certain embodiments described herein. While the discussion of themethod 100 herein refers to the structures schematically illustrated byFIGS. 2, 5, and 6, persons skilled in the art recognize that otherstructures are also compatible with embodiments described herein. In anoperational block 110, the method 100 comprises providing a frame 20comprising an edge connector 22 which is electrically connectable to thecomputer system. The frame 20 further comprises at least one layer ofthermally conductive material 50 which is thermally coupled to the edgeconnector 22. In an operational block 120, the method 100 furthercomprises mounting a first PCB 30 to the frame 20. The first PCB 30 hasa first surface 32 and a first plurality of components 34 mountedthereon. The components 34 are electrically coupled to the edgeconnector 22 and thermally coupled to the at least one layer ofthermally conductive material 50. In an operational block 130, themethod 100 further comprises mounting a second PCB 40 to the frame 20.The second PCB 40 has a second surface 42 and a second plurality ofcomponents 44 mounted thereon. The components 44 are electricallycoupled to the edge connector 22 and are thermally coupled to the atleast one layer of thermally conductive material 50.

FIG. 8 is a flowchart of providing the frame 20 in the operational block110 in accordance with certain embodiments described herein. In anoperational block 112, a base PCB 60 is provided, wherein the base PCB60 comprises the edge connector 22, the at least one layer of thermallyconductive material 50, a plurality of electrical contacts 62 at a firstsurface 63 of the base PCB 60, and a plurality of electrical contacts 64at a second surface 65 of the base PCB 60. The electrical contacts 62,64 are electrically coupled to the edge connector 22. In an operationalblock 114, a first riser PCB 70 comprising a plurality of electricalcontacts 72 is coupled to the base PCB 60. In certain embodiments,coupling the first riser PCB 70 to the base PCB 60 compriseselectrically coupling the electrical contacts 72 of the first riser PCB70 to the plurality of electrical contacts 62 at the first surface 63 ofthe base PCB 60. In an operational block 116, a second riser PCB 80comprising a plurality of electrical contacts 82 is coupled to the basePCB 60. In certain embodiments, coupling the second riser PCB 80 to thebase PCB 60 comprises electrically coupling the electrical contacts 82of the second riser PCB 80 to the plurality of electrical contacts 64 atthe second surface 65 of the base PCB 60.

FIG. 9 is a flowchart of mounting the first PCB 30 to the frame 20 inthe operational block 120 in accordance with certain embodimentsdescribed herein. In an operational block 122, the first PCB 30 isprovided, wherein the first PCB 30 comprises a plurality of components34, a plurality of electrical contacts 36 along an edge 37 of the firstPCB 30, and a plurality of component contacts 38. The electricalcontacts 36 are electrically coupled to the components 34 through thecomponent contacts 38. In an operational block 124, the electricalcontacts 36 are electrically coupled to the electrical contacts 72 ofthe first riser PCB 70.

FIG. 10 is a flowchart of mounting the second PCB 40 to the frame 20 inthe operational block 130 in accordance with certain embodimentsdescribed herein. In an operational block 132, the second PCB 40 isprovided, wherein the second PCB 40 comprises a plurality of components44, a plurality of electrical contacts 46 along an edge 47 of the secondPCB 40, and a plurality of component contacts 48. The electricalcontacts 46 are electrically coupled to the components 44 through thecomponent contacts 48. In an operational block 134, the electricalcontacts 46 are electrically coupled to the electrical contacts 82 ofthe second riser PCB 80.

In certain embodiments, each PCB used to fabricate the module 10 (e.g.,the first PCB 30, the second PCB 40, the base PCB 60, the first riserPCB 70, and the second riser PCB 80) has fiducial marks or structureswhich fit into a jig or other framework to facilitate orienting the PCBsrelative to one another during fabrication. Examples of structurescompatible with embodiments described herein include, but are notlimited to, notches, ridges, pins, and holes. FIGS. 11A-11Cschematically illustrate exemplary PCBs with holes 150 which fit ontocorresponding pins of a jig (not shown). FIG. 11A schematicallyillustrates a first PCB 30 with a plurality of holes 150 at selectedpositions. FIG. 11B schematically illustrates a first riser PCB 70 witha plurality of holes 150 at corresponding positions. FIG. 11Cschematically illustrates a base PCB 60 with a plurality of holes 150 atcorresponding positions. Similarly, each of the second PCB 40 and thesecond riser PCB 80 of certain embodiments has a plurality of holes 150at corresponding positions. Other embodiments have different numbers ofholes 150 at different positions than those schematically illustrated byFIGS. 11A-11C. Persons skilled in the art can select appropriate holesizes and positions in accordance with embodiments described herein.

FIG. 12 is a flowchart of an exemplary fabrication method 200 using ajig having pins corresponding to the holes 150 of the PCBs schematicallyillustrated by FIGS. 11A-11C. In an operational block 210, the first PCB30 is placed on the jig with the pins extending through the holes 150 ofthe first PCB 30. The first PCB 30 is placed on the jig with the firstsurface 32 facing upwards. In an operational block 220, the first riserPCB 70 is placed on the jig with the pins extending through the holes150 of the first riser PCB 70. The electrical contacts 72 of the firstriser PCB 70 are proximal to the electrical contacts 36 of the first PCB30. In an operational block 230, the base PCB 60 is placed on the jigwith the pins extending through the holes 150 of the base PCB 60. Theelectrical contacts 62 of the base PCB 60 are proximal to the electricalcontacts 72 of the first riser PCB 70. The at least one thermallyconductive layer 50 is thermally coupled to the components 34 of thefirst PCB 30. In certain embodiments, a thermally conductive grease isapplied between the components 34 of the first PCB 30 and the at leastone thermally conductive layer 50 prior to placing the base PCB 60 andthe first PCB 30 together. The thermally conductive grease of certainembodiments advantageously facilitates thermal coupling between thecomponents 34 and the at least one thermally conductive layer 50 of theframe 20.

In an operational block 240, the second riser PCB 80 is placed on thejig with the pins extending through the holes 150 of the second riserPCB 80. The electrical contacts 82 of the second riser PCB 80 areproximal to the electrical contacts 64 of the base PCB 60. In anoperational block 250, the second PCB 40 is placed on the jig with thepins extending through the holes 150 of the second PCB 40. Theelectrical contacts 46 of the second PCB 40 are proximal to theelectrical contacts 82 of the second riser PCB 80. The second PCB 40 isplaced on the jig with the second surface 42 facing downwards. The atleast one thermally conductive layer 50 is thermally coupled to thecomponents 44 of the second PCB 40. In certain embodiments, a thermallyconductive grease is applied between the top components 44 of the secondPCB 40 and the at least one thermally conductive layer 50 prior toplacing the base PCB 60 and the second PCB 40 together. The thermallyconductive grease of certain embodiments advantageously facilitatesthermal coupling between the components 44 and the at least onethermally conductive layer 50 of the frame 20.

In an operational block 260, the electrical contacts of the various PCBsare electrically coupled together to provide electrical conductivitybetween the edge connector 22 and the components 34, 44. In anoperational block 262, the electrical contacts 36 of the first PCB 30are electrically coupled to the electrical contacts 72 of the firstriser PCB 70. In an operational block 264, the electrical contacts 72 ofthe first riser PCB 70 are electrically coupled to the electricalcontacts 62 of the base PCB 60. In an operational block 266, theelectrical contacts 64 of the base PCB 60 are electrically coupled tothe electrical contacts 82 of the second riser PCB 80. In an operationalblock 268, the electrical contacts 82 of the second riser PCB 80 areelectrically coupled to the electrical contacts 46 of the second PCB 40.

Examples of methods of electrically coupling the respective electricalcontacts include, but are not limited to, edge-bonded interconnects (asdescribed more fully below), through-hole interconnects, male-femaleconnections, J-clips, and flex circuitry. Persons skilled in the art canselect appropriate methods of electrically coupling the respectiveelectrical contacts in accordance with embodiments described herein.

In particular, through-hole interconnects suffer from various problems.For example, solder joints used to provide the interconnection arelocated between the two PCBs, so the solder joints are not visible andare not accessible for visual inspection. In addition, the through-holeinterconnects add to the cost of manufacturing the module 10. Inaddition, the through-hole interconnects do not provide reliableelectrical interconnections between the two PCBs.

In certain embodiments utilizing edge-bonded interconnects, each of thefirst riser PCB 70 and the second riser PCB 80 has plated contacts inproximity to an edge of the PCB (e.g., either on the edge or cut intothe edge, as schematically illustrated by FIG. 3C). In certain suchembodiments, each of the operational blocks 262, 264, 266, 268 areperformed by applying solder to the plated contacts and reflowing thesolder using localized heating. By using localized heating, certain suchembodiments advantageously avoid exposing the components 34, 44 toadditional heat cycling, thereby reducing the probability of degradationor failure of the components 34, 44.

FIG. 13 schematically illustrates a side view of exemplary electricalconnections between the electrical contacts 62 of the base PCB 60, theelectrical contacts 72 of the first riser PCB 70, and the electricalcontacts 36 of the first PCB 30 using edge-bonded interconnects. Afterapplying solder 160 and reflowing the solder 160, the plated electricalcontacts 62, 72, 36 in proximity to the edge of the PCBs are wetted bythe solder 160, as schematically illustrated by FIG. 13. The edge-bondedinterconnects of certain embodiments provide vertical connectionsbetween two PCBs. In certain embodiments, applying the solder 160 to theoutside surfaces of the electrical contacts 62, 72, 36 advantageouslypermits visual inspection of the resultant electrical connections,thereby avoiding techniques such as x-ray analysis. The electricalconnections between the electrical contacts 62 of the base PCB 60 andthe electrical contacts 36 of the first PCB 30 are advantageouslyfacilitated in certain embodiments by the form of the electricalcontacts 72 of the first riser PCB 70, as schematically illustrated byFIG. 3C. Such structures for the electrical contacts 72 provide recessesinto which the solder 160 advantageously reflows upon localized heating.In certain embodiments, the edge-bonded interconnects advantageouslysimplify the module and board design, and reduce the cost ofmanufacturing the module and the board. By providing interconnects whichcan be visually inspected, certain embodiments utilizing edge-bondedinterconnects advantageously facilitate identification of poorinterconnections which can be rejected or reworked.

As described above, in certain embodiments, the at least one thermallyconductive layer 50 comprises two thermally conductive layers 50 a, 50 bon either side of a dielectric layer 61. FIGS. 2, 5, and 6 schematicallyillustrate such embodiments. In certain embodiments in which the twothermally conductive layers 50 a, 50 b are also electrically conductive,the two thermally conductive layers 50 a, 50 b are thermally andelectrically coupled to the same electrical contacts 22 of the edgeconnector 24. In certain other embodiments, the two thermally conductivelayers 50 a, 50 b are thermally and electrically coupled to two separatesets of the electrical contacts 22 of the edge connector 24. Thus, incertain such embodiments, the two thermally conductive layers 50 a, 50 bare electrically isolated from one another.

In certain embodiments, one thermally conductive layer 50 a is thermallyand electrically coupled to the electrical contacts 22 corresponding toa ground plane while the other thermally conductive layer 50 b isthermally and electrically coupled to the electrical contacts 22corresponding to a voltage plane. In certain embodiments, the frame 20comprises between approximately twenty to thirty electrical contacts 22to ground and between approximately twenty to thirty electrical contacts22 to a power voltage. Thus, the number of electrical contacts 22 usedto provide the thermal path is advantageously increased by connectingthe thermally conductive layers 50 a, 50 b to different sets ofelectrical contacts 22. Certain such embodiments advantageously providea degree of electromagnetic interference (EMI) shielding of thecomponents 34, 44 of the module 10. Certain other such embodimentsadvantageously provide capacitance between the two thermally conductivelayers 50 a, 50 b which facilitates noise reduction of the voltageapplied to the voltage plane.

In certain embodiments, the module 10 further comprises a thermallyconductive piece 170 which is positioned on the module 10 along aportion of the opposite edge away from the edge connector 22, asschematically illustrated by FIG. 14. The piece 170 is thermally coupledto the at least one thermally conductive layer 50 and provides a secondthermal path for heat to transfer to the piece 170 from the components34, 44, through the at least one thermally conductive layer 50. The heatcan then be dissipated away from the piece 170 by convection to theenvironment surrounding the module 10.

FIG. 15 schematically illustrates an exemplary module 10 having a firstframe portion 180 and a second frame portion 182 in accordance withembodiments described herein. The at least one thermally conductivelayer 50 extends from the first portion 180 of the frame 20 to thesecond portion 182 of the frame 20. Each of the first PCB 30 and thesecond PCB 40 is coupled to both the first portion 180 of the frame 20and the second portion 182 of the frame 20 (e.g., by solder balls 184).The at least one thermally conductive layer 50 is sandwiched between thecomponents 34 of the first PCB 30 and the components 44 of the secondPCB 40.

Various specific embodiments have been described above. Although thepresent invention has been described with reference to these specificembodiments, the descriptions are intended to be illustrative of theinvention and are not intended to be limiting. Various modifications andapplications may occur to those skilled in the art without departingfrom the true spirit and scope of the invention as defined in theappended claims.

1. A module electrically connectable to a computer system, the modulecomprising: a frame having an edge connector with a plurality ofelectrical contacts which are electrically connectable to the computersystem; a first printed circuit board coupled to the frame, the firstprinted circuit board having a first surface and a first plurality ofcomponents mounted on the first surface, the first plurality ofcomponents electrically coupled to the electrical contacts of the edgeconnector; a second printed circuit board coupled to the frame, thesecond printed circuit board having a second surface and a secondplurality of components mounted on the second surface, the secondplurality of components electrically coupled to the electrical contactsof the edge connector, the second surface of the second printed circuitboard facing the first surface of the first printed circuit board; andat least one thermally conductive layer positioned between the firstplurality of components and the second plurality of components, the atleast one thermally conductive layer thermally coupled to the firstplurality of components, to the second plurality of components, and tothe electrical contacts of the edge connector.
 2. The module of claim 1,wherein the at least one thermally conductive layer comprises copper. 3.The module of claim 2, wherein the layer has a thickness ofapproximately 0.2 millimeter.
 4. The module of claim 2, wherein the atleast one thermally conductive layer comprises a two-ounce copper sheet.5. The module of claim 1, wherein the thermally conductive layerconducts heat away from the first plurality of components and from thesecond plurality of components to the computer system through theelectrical contacts of the edge connector.
 6. The module of claim 1,wherein the module has a thickness of less than approximately 5.6millimeters.
 7. The module of claim 1, wherein the first printed circuitboard and the second printed circuit board are stacked and are generallyparallel to one another.
 8. The module of claim 1, wherein thecomponents comprise memory integrated circuits.
 9. The module of claim8, wherein the memory integrated circuits comprise dynamic random-accessmemory (DRAM) integrated circuits.
 10. The module of claim 9, wherein atleast some of the memory integrated circuits are stacked.
 11. A moduleconnectable to a computer system, the module comprising: a framecomprising an edge connector connectable to the computer system; a firstprinted circuit board coupled to the frame, the first printed circuitboard having a first surface and a first plurality of components mountedon the first surface, the first plurality of components electricallycoupled to the edge connector; a second printed circuit board coupled tothe frame, the second printed circuit board having a second surface anda second plurality of components mounted on the second surface, thesecond plurality of components electrically coupled to the edgeconnector, the second surface facing the first surface; and a heatspreader comprising at least one sheet of thermally conductive material,the heat spreader positioned between and thermally coupled to the firstplurality of components and the second plurality of components, the heatspreader thermally coupled to the edge connector.
 12. The module ofclaim 11, wherein the thermally conductive material comprises copper.13. The module of claim 12, wherein the sheet has a thickness ofapproximately 0.2 millimeter.
 14. The module of claim 12, wherein theheat spreader comprises a two-ounce copper sheet.
 15. The module ofclaim 11, wherein the heat spreader conducts heat away from the firstplurality of components and from the second plurality of components tothe computer system through the edge connector.
 16. The module of claim11, wherein the first printed circuit board and the second printedcircuit board are stacked and are generally parallel to one another. 17.The module of claim 11, wherein the components comprise memoryintegrated circuits.
 18. The module of claim 17, wherein the memoryintegrated circuits comprise dynamic random-access memory (DRAM)integrated circuits.
 19. The module of claim 18, wherein at least someof the memory integrated circuits are stacked.
 20. A method ofconducting heat away from a first plurality of components mounted on afirst surface of a first printed circuit board and from a secondplurality of components mounted on a second surface of a second printedcircuit board, the method comprising: coupling the first printed circuitboard and the second printed circuit board to a frame comprising an edgeconnector, the first surface facing the second surface; positioning athermally conductive layer between the first plurality of components andthe second plurality of components; thermally coupling the thermallyconductive layer to the first plurality of components, to the secondplurality of components, and to the edge connector; and electrically andthermally coupling the edge connector to a computer system, therebyproviding a thermal pathway for heat to be removed from the firstplurality of components and from the second plurality of components tothe computer system through the edge connector.
 21. A method offabricating a module electrically connectable to a computer system, themethod comprising: providing a frame comprising an edge connectorelectrically connectable to the computer system, the frame furthercomprising at least one layer of thermally conductive material which isthermally coupled to the edge connector; mounting a first printedcircuit board to the frame, the first printed circuit board having afirst surface and a first plurality of components mounted on the firstsurface, the first plurality of components electrically coupled to theedge connector and thermally coupled to the at least one layer ofthermally conductive material; and mounting a second printed circuitboard to the frame, the second printed circuit board having a secondsurface and a second plurality of components mounted on the secondsurface, the second plurality of components electrically coupled to theedge connector and thermally coupled to the at least one layer ofthermally conductive material.